Check valve in the hydraulic circuit is widely used, but the valve should be fully consider the characteristics of the valve, otherwise it will cause the loop is not stable, especially in high-pressure system, the pilot-operated check valve open reverse The pressure at the front port P2 is high, as shown in Figure 1a. To reduce the control pressure, hydraulic shock and noise in the hydraulic system, a hydraulic controlled check valve is also used which unloads the spool. The unloader valve allows the pilot Check valve performance is improved, the pilot check valve is characterized when the pilot check valve port K is no pressure oil, the pressure oil can only flow from port P1 to port P2, can not be reversed. When the control port K has control pressure oil, the oil pushes the ejector rod due to the control piston, and the ejector pin opens the unloading valve spool at the same time. At the same time, the main valve spool opens and connects the two ports of P1 and P2. The oil can be in these two Free flow direction, the unloading valve reduces the hydraulic impact, hydraulic check valve performance more stable, but there is still a large flow of high-pressure impact or noise. Hydraulic check valve is often used in packing, pressure relief, locking circuit. As the pressure in the system, due to pressure, oil compression, pipe expansion, elastic deformation of components can be stored energy, the pressure relief, the amount of kidney suddenly release, and the valve from the median to the left position , Due to the pressure on the cylinder cavity is large, then the lower chamber pressure has risen, unloading valve and the main valve is almost opened at the same time, so that the pressure on the cavity suddenly through the hydraulic check valve to the oil return road. Due to these two reasons, the pressure relief after packing pressure is too fast, resulting in shock vibration and noise, for which we have improved this type of hydraulic circuit. As the high-pressure high-flow hydraulic check valve caused the pressure relief too fast, so that the impact increases, so add a throttle in the loop, so that the pilot-way unloader valve unloading valve to open, and then open The main valve, due to the throttle valve has sufficient flow control range, to ensure a stable minimum flow, easy adjustment, small leaks, temperature and pressure on the impact is small, so by regulating the throttle valve to control the fluid in the upper chamber Pressure relief time is also extended accordingly, the pressure will enhance the ability to send back on the hydraulic oil on the impact of various components and noise.